The present invention relates to a fixing device provided in a copier, a printer, a facsimile, and the likes, and to an image forming apparatus having the fixing device.
A conventional image forming apparatus includes a fixing device using an endless belt for reducing power consumption or shorting a start-up time (refer to Patent Reference). Patent Reference: Japanese Patent Publication No. 2006-154823
FIG. 9 is a schematic view showing a conventional fixing device using an endless belt. As shown in FIG. 9, the conventional fixing device includes a pressing roller 2′ formed of a shaft metal 2a′ and an elastic layer 2b′. The pressing roller 2′ receives a rotational drive force from a drive source (not shown) through the shaft metal 2a′. 
Further, the conventional fixing device includes a fixing roller 3′ formed of a shaft metal 3a′ and an elastic layer 3b′. The pressing roller 2′ presses the fixing roller 3′ to form a nip portion N. The conventional fixing device further includes a fixing belt 4′ or an endless belt having a cylindrical shape placed between the fixing roller 3′ and a supporting member 5′, and a heat source 6′ having a plate shape disposed in the fixing belt 4′.
In the conventional fixing device described above, the drive source (not shown) drives the pressing roller 2′ to rotate. When the pressing roller 2′ rotates, the fixing roller 3′ follows the pressing roller 2′ to rotate through a frictional force between the pressing roller 2′ and the fixing belt 4′, and a frictional force between the fixing belt 4′ and the fixing roller 3′. When the fixing roller 3′ rotates, the fixing belt 4′ moves against a frictional force between the fixing belt 4′ and the supporting member 5′.
In the conventional fixing device described above, the heat source 6′ maintains the fixing belt 4′ at a specific temperature. To this end, the fixing belt 4′ is formed of a material with high heat resistance and a low thermal conductivity. The fixing belt 4′ maintained at the specific temperature moves a recording sheet (not shown) to pass through the nip portion N. Accordingly, un-fixed toner is heated and pressed on the recording sheet for fixing. The pressing roller 2′ presses the fixing roller 3′ to form the nip portion N, so that a sufficient amount of heat is supplied to the recording sheet and un-fixed toner.
In the conventional fixing device described above, when the fixing belt 4′ maintained at the specific temperature moves the recording sheet to pass through the nip portion N, the recording sheet (a medium) absorbs heat from the fixing belt 4′ at a portion thereof where the recording sheet contacts with (a medium passing portion) corresponding to a width of the recording sheet perpendicular to a direction that the recording sheet is transported. On the other hand, the recording sheet does not absorb heat from the fixing belt 4′ at other portion thereof where the recording sheet does not contact with (a medium non-passing portion) corresponding an area outside the width thereof.
Further, when the fixing belt 4′ is formed of a material having a low thermal conductivity, a temperature difference is generated between the medium passing portion and the medium non-passing portion. As a result, when the media are transported continuously, the medium non-passing portion is excessively heated, thereby making it necessary to wait until a temperature thereof decreases.
In view of the problems described above, an object of the present invention is to provide a fixing device and an image forming apparatus capable of solving the problems of the conventional fixing device.
Further objects and advantages of the invention will be apparent from the following description of the invention.